57 



most vioient when the sheep have been heated by driving or warming 

 in a stable. 



Pathology. ^Bj separating the wool and examining a recently infected 

 spot, there can be seen some minute elevations, which differ from the 

 surrounding skin in being slightly whiter or yellower, and which have 

 been produced by the bites of the pests. The insects themselves can 

 be found among the hairs at but little distance from the bites. As time 

 passes and the insects multiply in numbers these elevations become 

 more and more numerous, and closer and closer together, until they 

 finally unite over a considerable extent. Prom the summit of each eleva- 

 tion or papule, a watery, serous fluid exudes and accumulates, which 

 transforms them into vesicles and pustules, and which in drying cover 

 them over with a thin crust. In a few days the whole surface is cov- 

 ered with a yellowish, greasy, scaly layer, under which the parasites 

 are hidden. As the disease proceeds this layer gradually increases in 

 thickness by an increase of the serous exudate, and in circumference 

 by the extension of inflammation produced by the ever-multiplying par- 

 asites which live beneath it, forming scaly crusts. These crusts, in be- 

 ing torn out, mainly by the rubbing with which the sheep endeavors to 

 allay its intense itching, carry with them the lags of the wool, the loss 

 of which is an early symptom of the disease. At a later period the 

 crusts are replaced by another set of thicker, firmer, adherent scabs, 

 which are still further enlarged by the outward migration of the para- 

 sites. As they abandon the center of the scabs these are again replaced 

 by a peeling off of the external layers of the skin, which gradually 

 heals, while the disease slowly progresses at the outside. The complete 

 cure is very slow, and the skin remains thick and folded for a long 

 time. In sheared sheep the skin becomes covered by a thick, dry 

 crust, like.parchment, while beneath it remains much swollen. 



Late symptoms and diagnosis. — The fleece of scabby sheep presents a 

 characteristic rough look. In places the wool is stuck together in 

 masses; in others it fails, while in others, which are apparently sound, 

 it can be easily plucked off. The rubbing and scratching indulged in 

 by the sheep not only tend to tear away tlie wool but increase the 

 irritation of the skin, which may become intensely inflamed and swollen 

 and finally end in a superficial death of the part. Unlike Sarcoptes, 

 the Psoroptes seeks the longest, thickest wool. It begins its attack along 

 the back and extends to the neck, flanks, and rump. The Psoroptes 

 are rarely found in the region of the chest and abdomen. They are 

 collected in masses on circumscriGed silrfaces. The scabs they produce 

 constantly increase at their edges, and their number depends on the 

 number of places invaded. Owing to the closeness in which sheep con- 

 gregate and to their violent scratching the parasites become very gen- 

 erally scattered and finally the scabs may run together. 



While few of the parasites are present in the older diseased parts, at 

 the edges of the scabs they can be found in swarms. They look like 



